Conventional processes for styrene manufacture which are in general commercial use employ ethylbenzene as the starting material or immediate precursor of the styrene product. In a large majority of these processes, the ethylbenzene is catalytically dehydrogenated to yield the desired styrene product. Typically, the conversions of ethylbenzene to styrene obtained with these processes is far from complete, typically at a rate of about 50-70% per pass across the reactor. Therefore, in normal operations, the dehydrogenation reaction product will be a mixture containing substantial portions of styrene and ethylbenzene as well as minor amounts of reaction by-products and impurities such as benzene, toluene, light ends including hydrogen, methane and ethylene, and heavy ends. The unreacted ethylbenzene must then be recovered and separated from the styrene product prior to recycle to the dehydrogenation reaction system. Thus, the mixture of light components, ethylbenzene, styrene and heavies is typically fed to a distillation train for SM product purification and EB recovery. The general practice is to accomplish these purifications by distillation, as taught for example in U.S. Pat. No. 3,904,484 (King) which patent is incorporated herein by reference.
The separation of the desired styrene product from the light ends, heavy ends, benzene and toluene is relatively easy, being accomplished by conventional sequential distillations. To separate the various components, the distillation section of a styrene plant will typically consist of at least three independent column systems. The column first in the series recovers the light components such as benzene and toluene (B/T Column), the second column recovers unreacted ethylbenzene (EB/SM Column), and the last column distills heavies from the finished styrene product (Finishing Column). Separation by distillation of the styrene monomer (SM) from the unreacted ethylbenzenie (EB), however, presents a considerably more difficult problem due primarily to their close similarity in volatility. First of all, the boiling points of ethylbenzene and styrene, 136.15.degree. C. at 760 mm Hg and 146.0.degree. C. at 760 mm Hg, respectively, are so close as to make sepertion by fractional distilled difficult. Conventionally, this EB/SM separation has been accomplished by distillation under vacuum conditions in large, sophisticated, and expensive distillation columns due to the large number of theoretical plates required to effect a good separation. Thus, conventionally, unreacted ethylbenzene from the dehydrogenation reaction section is separated from styrene in a single distillation column. In the standard design, a large number of theoretical stages (between 85 and 100) is required to effect the required separation. This single unit operation accounts for between 70 and 80 percent of the total distillation section heat input. In a typical plant, the separation of unreacted ethylbenzene from styrene product accounts for approximately 20-30% of the plant's steam consumption. If the energy consumption required for separating ethylbenzene from styrene in a 500,000 MTA styrene monomer plant could be reduced 50%, the savings would be on the order of $700,000/year.
Even under vacuum conditions, a polymerization inhibitor is added to the mixture because of the tendency of the styrene product to polymerize at the time and temperature conditions required to effect the separation by distillation. Styrene polymerizes to a measurable degree even at room temperature. The key which allows styrene distillation to be commercially practiced is the use of chemical additives referred to as polymerization inhibitors. To minimize styrene polymerization, and the associated fouling of equipment and need to process a highly viscous product stream, commercial styrene distillation is nearly always carried out under vacuum conditions (e.g., operating with a column overhead pressure of about 40 to 120 mm Hg abs). In the temperature range utilized by commercial styrene units, the rate of polymerization of uninhibited styrene doubles for every 10.degree. C. temperature increase. Also, to achieve the larger number of stages required to effect the separation, currently either structured or random dump packing materials are used as the internal vapor/liquid contacting medium. Packing materials intrinsically have much lower pressure drop compared to standard distillation trays. With packing, the lower pressure drop allows the column to operate with a comparatively lower bottoms temperature. As a result of these various process difficulties, costs, and limitations, however, considerable incentive has existed for many years to develop alternative means of effecting this separation which could be more viable from either or both economic and ease of operation standpoints. A number of patents have attempted to address these problems in a variety of ways.
Thus, U.S. Pat. No. 3,515,647 (Van Tassell et al.) teaches a process for purifying styrene via a distillation scheme having associated therewith a wiped wall thin film evaporator to maximize recovery of styrene from the residue material. Styrene in a purity of at least 99% by weight is recovered as a separate product stream.
In U.S. Pat. No. 3,702,346 (Kellar), a process for the steam dehydrogenation of ethylbenzene to styrene, the selectivity of the dehydrogenation reaction is improved by maintaining the reactor products settler, wherein condensed reactor products are separated, at a pressure less than atmospheric. This improvement in selectivity in turn somewhat reduces the costs and difficulties of the subsequent styrene separation.
U.S. Pat. No. 3,776,970 (Strazik et al.) describes a process in which styrene is separated from organic mixtures comprising styrene and ethylbenzene by contacting the mixture against one side of a polyurethane elastomer membrane under pervaporation permeation conditions and withdrawing at the other side a vaporous mixture having increased styrene concentration. The polyurethane elastomer contains polyether or polyester groupings.
U.S. Pat. No. 3,801,664 (Blytas) teaches another process in which styrene is separated from ethylbenzene in high yield and purity. The process comprises: (a) extraction with a two-phase solvent system in which the extracting phase is a concentrated anhydrous cuprous nitrate/propionitrile solution, wherein the styrene is selectively complexed with the cuprous ion, and the ethylbenzene countersolvent is a C.sub.5 to C.sub.18 paraffin; and (b) separation of the propionitrile solution phase containing the styrene-cuprous ion complex to recover the styrene therefrom.
U.S. Pat. No. 3,904,484 (King) describes a multi-stage distillation which involves fractionally distilling the dehydrogenation reaction effluent under subatmospheric pressure in a multistage distillation unit comprising a plurality of distillation stages to separately recover styrene monomer, unreacted ethylbenzene and by-product styrene tar residue comprising styrene polymers, C.sub.9+ aromatic hydrocarbons and polymerization inhibitors. The improvement claimed for this process involves recycling previously recovered styrene tar residue to the dehydrogenation reaction effluent at a point upstream of the separation of the styrene monomer and ethylbenzene so is to maintain a liquid volume ratio of 1 to 20 volumes of styrene tar residue to 20 to 1 volumes of reaction effluent, and distilling the dehydrogenation reaction effluent in the presence of the recycled styrene tar residue.
Others have recovered the overhead condensing duty (thermal energy) from the ethylbenzene/styrene distillation column by using it to boil an ethylbenzene/water azeotrope, for example in U.S. Pat. No. 4,628,136 (Sardina). Such a method requires a large heat transfer area and the use of a falling film evaporator, both of which require costly capital investments and entail costly maintenance. This method also links the dehydrogenation reaction section of the operation directly to the ethylbenzene/styrene splitter, which may not be desirable because upsets in the distillation section could result in difficulty in controlling the reactor section of this system and also could cause damage to the dehydrogenation catalyst.
All of the foregoing prior art processes for separating styrene product from unreacted ethylbenzene following a dehydrogenation reaction therefore have various disadvantages and drawbacks. High costs are incurred due to equipment requirements, maintenance and operating expenses in these prior art processes. These and other drawbacks with and limitations of the prior art processes are overcome, in whole or in part, with the cascade reboiling apparatus and process of this invention.